THE EVOLUTION OF LAURENTIA AS DOCUMENTED BY 40AR/39AR THERMOCHRONOLOGY STUDIES

The southern margin of Laurentia experienced a long-lived and punctuated convergent tectonic history between ca. 1.8 to 1.0 Ga that can be examined with geo and thermochronological data. Hundreds of new 40Ar/39Ar dates from Precambrian rocks of the Rocky Mountains and the southwestern USA result in a complex distribution of apparent ages that reflect the integrated effect of periods of long-term mid- crustal residence interrupted by periods of magmatism, metamorphism, and exhumation along discrete fault networks. Much of New Mexico and central Colorado record less than or equal to 1.45 to 1.3 Ga hornblende and mica dates that are independent of the spatial distribution of 1.4 Ga plutons. We suggest that these dates could record cooling from a regional 1.4 Ga thermal event. High regional temperatures could, in part, result from addition of a basaltic underplate at the base of the crust that increased the conductive geotherm and generated magmas that advected heat into the mid-crust. Ages older than ca. 1.4 Ga occur mainly north of the Cheyenne belt and also as “islands” where structurally high and cool rocks were not heated above ca. 300°C at 1.4 Ga. Ages less than 1.3 Ga mainly record regionally variable exhumation that appears to be controlled by a complex polygonal network of ~10 km scale blocks that were differentially exhumed between ~1.4 Ga to 0.5 Ga. 40Ar/39Ar K-feldspar data indicate at least two major episodes of exhumation that coincide with the ca. 1.1 Ga Grenville orogeny and the ca. 800 Ma rifting of Rodinia. Also, in places where K-feldspar record overall young ages, micas are young as well and this supports the idea that variable paleodepths are preserved beneath the Great Unconformity. K-feldspar data also document that some Ancestral Rocky and Laramide fault networks were initiated in the Precambrian. The exhumation history documented by thermochronological data will play a key role in developing geodynamic models to describe intracratonic deformation histories in southwestern Laurentia.